Conventional UHF radio systems universally use frequency modulation (FM) for speech transmission purposes and frequency or phase shift keying (FSK or PSK) for the transmission of other data. Frequency channel spacing in such systems at frequencies above 300 MHz is usually at least 25 KHz, the actual spacing being dictated by the stability requirements for the local receiver and transmitter oscillators. The system must remain operative within an acceptable tolerance of the nominal channel frequency for a range of different supply voltages, temperatures and over a predetermined time from the initial setting of the transmitter/receiver. Although some retuning of the transmitter/receiver has been acceptable in the past, nowadays it is highly desirable that the equipment remains on frequency for life (e.g. five years). Some exemplary figures illustrating the problem of frequency shift in UHF radio systems are as follows: (a) If a total acceptable shift for frequency modulation (FM) or frequency shift keying (FSK) operation is 1 KHz and the transmission frequency is 1 GHz then the total frequency shift from all causes over a period of 5 years will be one part in one million. This represents the frequency shift limit achievable with good quality crystals for frequency control.
When it is further realised that this very small frequency shift limit must also include the transmit/receive station offsets and spread between equipments the shift limit is clearly very stringent. Consequently, more bandwidth efficient systems such as very narrow band multi-channel FM (e.g. channel spacing of 12.5 KHz or 6.25 KHz) systems would likely be impractical and single sideband systems which require even better stability would definitely not be possible. There is no real prospect of significant improvement in frequency control crystal stability and although more expensive control crystals are more stable their stability is improved only by a very small factor.